1. Field of the Invention
The present invention relates to an apparatus for processing image data and more particularly to an apparatus which processes image data read by an image reader.
2. Description of Prior Art
A document is put on a platen glass when it is read by an image reader. When the document is put on the platen glass at an erroneous position, it is desirable to correct the tilt of the document. Then, a function is proposed to rotate 2-dimensional image data obtained by a document reader by the tilt angle of the document (for example, Japanese Patent Publication 63-9266/1988). In the tilt angle rotation, inclination of a document put on a platen glass is detected, and the image data are rotated to the normal position. Then, an image is formed on a paper based on the corrected image data. Thus, when a user put a document at a tilt unintentionally, its image can be output by automatically correcting the tilt of the image.
However, in the prior art tilt correction by rotating the image data by the tilt angle, the image is processed automatically in the same process regardless of a simple bi-level image such as a character image or a half-tone image such as a photograph. Further, the processing is similar irrespective of the magnitude of the rotation angle. For a character image, the image after the tilt angle rotation keeps about the same image quality (for a resolution of about 400 dots per inch). On the other hand, for a half-tone image, the image quality is deteriorated disadvantageously after the rotation. For example, the image becomes rough at half-tone image areas. Especially, the deterioration becomes worse if the rotation angle becomes larger, for example more than about 10.degree.. It is to be noted that it is difficult to use a single criterion for the tilt angle rotation, because half-tone images include various kinds and user's purposes for image forming are also different.
If multi-level image data (data having gradation) are subjected to the tilt angle rotation, image deterioration on the tilt angle rotation is suppressed to some degree. However, this needs a complicated structure, and the processing speed becomes slow. Further, the cost increases to a large extent.
For a half-tone image, high image quality is required, and smoothing or the like is needed in the data modification.
However, for a document including half-tone and character images mixed with each other, smoothing for a character image (simple bi-level image) may worsen image quality reversely. When an image is formed, it is convenient for a user to automatically correct the tilt of a document, but deterioration of image quality is a problem.
Further, for a document including half-tone and character images mixed with each other, deterioration of image quality is less for a character image (simple bi-level image) than for a half-tone image. Thus, the image quality subjected to the rotation becomes different between the character image areas and the half-tone image areas. When such an image formed on a sheet of paper is viewed, the entire image seems not well.
In a prior art tilt correction technique, it is known to combine image data shift in two scan directions with 90.degree. rotation. In order to prevent size change of the image, electric magnification is performed after the shifting in each scan direction. However, when electric magnification is performed for each shifting, line insertion or the like happens by the electric magnification, and this deteriorates the image. Further, processing time for the rotation becomes longer.